Techniques for Generation Techniques for Generation of Terahertz Radiationof Terahertz Radiation

E.V.Suvorov

Institute of Applied Physics of Russian Academy of Sciences46, Uljanov Str., 603950, Nizhny Novgorod, Russia

FNP – 2007 July 3 – 9, 2007“Georgy Zhukov”N.Novgorod – Saratov - N.NovgorodRussia

OUTLINE

♦ Motivation

♦ Generation by means of vacuum electronics

♦ Generation by means of “optoelectronics”

♦ “Exotic” ways

♦ Conclusions

TT--RayRay: : Next frontier in Science Next frontier in Science and Technologyand Technology

103 106 109 1012 1015 1018 1021 1024100

MF, HF, VHF, UHF, SHF, EHFmicrowaves visible

kilo mega giga tera peta exa zetta yotta

x-ray γ -ray

THz Gapelectronics photonics

Hz

Frequency (Hz)

1 THz ~ 1 ps ~ 300 µm ~ 33 cm-1 ~ 4.1 meV ~ 47.6 oK

dc

Terahertz wave (or T-ray), which is electromagnetic radiation in a frequency interval from 0.1 to 10 THz, lies a frequency range with rich science but limited technology.

APPLICATIONS

Spectroscopy: Chemistry, Aeronomy, Ecology,Radioastronomy, …

Tera-imaging: Biology, Biomedicine, Microelectronics, Technology, Security, …

Plasma diagnostics: Interferometry, Faraday, Cotton-Mauton, …

…

Vacuum electronics

♦Cherenkov generation (BWOs, TWTs, Orotrons)

♦Transition generation (Klystrons)

♦Bremsstrahlung (gyrodevices, FELs)

♦Scattering generation

evgr

Pin Pout

e

vgrPout

e

vgr

Pout

Cherenkov generation

TWT BWO

Orotron, or Diffraction Radiation Generator

dh π2

=

υω h=

πβγλ2

122

=−

=Λ ⊥kh

β = υ/c

1.8x3.61.8x3.61.8x3.61.8x3.61.8x3.61.8x3.61.2x2.41.2x2.4Output waveguide

11111111101097Guiding magnetic field, kOe

30 - 4530 - 4530 - 4530 - 4530 - 4530 - 4525 - 4025 - 40Cathode current, mA

1.5 – 6.01.5 – 6.01.5 – 6.01.5 – 6.01.5 – 6.01.5 – 6.01.0 – 5.01.0-4.0Acc. Voltage, kV

1313131313131313Power variation (over the band), dB

0.5 - 20.5 - 20.5 - 30.5 - 31 - 51 - 51 - 51 - 10Output power (min), mW

1170 -1400

1070 -1200

900 -1100

790 -970

690 -850

530 -714

370 -535

258 -375

Band, GHz

OB-85*OB-84*OB-83OB-82OB-81OB-80OB-32OB-30Tube

•Temporarily not produced

Submm TWTs have been also designed

Commercial BWOs (“ISTOK”, Fryazino, Russia)

Clinotron variety of BWO (Kharkov Institute of Radio Astronomy)

mm-wave clinotron submm-wave clinotron

heatpipe3.01604.05.082-96CTN-3MT

liquid122005.50.05-0.1442-510CTN-0.5M8

liquid121605.00.1345-390CTN-0.8M8

liquid1.21404.52.0137-151CTN-2.0M3

liquid1.21604.52.0120-141CTN-2.2M3

liquid1.21804.33.0113-122CTN-2.5M3

liquid1.21505.05.079-98CTN-3M3

liquid1.22004.011.053-63CTN-5M3

CoolingWeight, kgMax. AnodeCurrent, mA

Max. AnodeVoltage, kV

Max OutputPower, W

Frequencyband, GHzModel

LOW-VOLTAGE OROTRONS

Slow-wave structure creates spatial harmonics of cavity mode. The first harmonic is in synchronism with electrons:

vd

vh πω 21 ==

Amplitude of the synchronous harmonic decreases at the distanceSmall part of electrons moving over the structure interacts with the wave.

In order to avoid it, electrons move inside a multiple-rod structure!

π2d

=Λ

Rods: 20 μm × 50 μm × 500 μm. Main problem: manufacturing the structures.

Cherenkov oscillator with open cavity and reflecting grating (F.S. Rusin, G.D. Bogomolov)“Diffr. Rad. Generators” (V.P. Shestopalov et al.)

OROTRONS IN IAP AND GYCOM

Thermionic cathode 3 mm×0.3 mm.Current density 30 A/cm2.

Two-mirror open cavity with output Waveguide (Q~3,000 - 8,000).

Multiple-rod periodic structure.

Packaged with permanent magnets (1.25 T, 23 kg).

Electronic and mechanical frequency tuning.

In collaboration with Institute of Metrology of Time and Spaceand Institute of Spectroscopy

50ns-1ms50ns-1ms50ns-1msPulse duration

0.020.020.03Fine frequency tuning, %

10-610-610-6Frequency stability

< 250< 300< 200Electron current, mА

1.1 ÷ 4.00.6 ÷ 3.70.8 ÷ 3.0Voltage, kV

100120170Period of structure, μm

60 ÷ 100100 ÷ 200200 ÷ 1000Output power, mW

200 ÷ 370120 ÷ 300100 ÷ 190Frequency band, GHz

OR-360OR-290OR-180

Submm-wave gyrodevices

1. Conventional gyrotrons (n = 1,2)a) CW gyrotrons with frequency up to 600 GHzb) gyrotrons with strong pulsed magnetic fields

2. Large-Orbit Gyrotrons (LOGs)

3. Frequency multipliers

Strong magnetic field OR – high cyclotron harmonicsOR – both

f (THz) ≈ n (B / 36 T)

300GHz/4kW/CW Gyrotron (n=1); V.Zapevalov, e.a., 2005MAGNETIC SYSTEM (12T LHe-free SC magnet)

Collaboration with FIR Center FU

7.28 7.32 7.36 7.40 7.44

0.00

0.02

0.04

0.06

0.08η

0

200

400

600

800

1000

η

q=2

q=1

, P

q=2

q=3

q=4

TE 6.5 , 400 GHz , n=2 , L=20 mm

Uo=15 kV , I=0.8 A , g=1.3 , dv =0.3TQ=8400

Q=4415

P , W

Bo , T

10 15 20

0.0

0.1

0.2

0.3

0.4

η

η

g=1.4

g=1.2

g=1

0

2

4

6

8

g=1

g=1.2

g=1.4

P

P , kWTE 22.8 , 300 GHz , L=17

I=1 A , dv =0.3T

Uo , kV

Project 400GHz/0.2kW/CW Gyrotron (n=2)

M.I.Petelin et al., 1974: 330GHz/1.5kW/CW Gyrotron (n=2)

1THz / 1.5kW / 50μs Gyrotron (n=1)PULSED MAGNETIC FIELD (40T, LN-cooling system)

M.Glyavin, e.a., 2007

1THz/0.5kW/100 μs Gyrotron (n=2)PULSED MAGNETIC FIELD (20T magnet)

M.Glyavin, e.a., 2005

V.Flyagin, e.a., 1983: 0.65THz/40kW/50μs Gyrotron (n=1)

ATTRACTIVITY OF HIGH CYCLOTRON HARMONICS IN LARGE ORBIT GYROTRONS

Resonance interaction of a gyrating electron with the synchronously rotating electric multipole (2s-pole)

Perfect selection over azimuthal index!

cs Ω≈ω

370-414GHz/10kW/10μs LOG (n=3)PULSED MAGNETIC FIELD (7T magnet)

0.4 0.8 1.2 1.6 2current (A)

0

2

4

6

8

10

pow

er (k

W)

0

0.4

0.8

1.2

1.6

2

effic

ienc

y %

η

P

Large orbit gyrotronsV.Bratman, e.a., 2005

Projects of 3rd-harmonic LOGs with cusp guns: 80 keV/0.7 A/10μs 13.6 T TE3,5 1 THz 1 kW30 keV/ 1 A/ CW 7 T TE3,5 0.6 THz 0.5 kW

FREQUENCY MULTIPLICATIONDevelopment of old idea: Gyromultiplier without external signal

V.Bratman, G.Denisov, e.a.,2005

LF section: self-generation at the fundamental cyclotron harmonic N =1 at the frequency ω

HF section: the bunched electron beam radiates the HF wave at a multiplyed frequency, Nω , and at the high cyclotron harmonic, N

95/285 GHz gyrotron with frequency multiplication

ω1≈ ωc

n=3, Ist>I0

ω2= 3ω1

n=1, Ist<I0

FINAL GOAL: to obtain THz radiation in a third harmonic CW compact gyrotron, operating in 10T cryomagnet

PROBLEMS OF CONVENTIONAL GYROTRONS:high starting current of operating modestrong mode competition high ohmic loses in RF circuit

PROPOSED SOLUTION:two-cavity gyrotron with frequency multiplication. Its first cavity is self - excited at

the first cyclotron harmonic. Modulated and bunched electron beam excites forced oscillations in the output cavity at triple frequency .

95/285 GHz gyrotron with frequency multiplication. Preliminary Test results.

Output power and frequency of oscillations in the first cavity vs. its length at U=23kV, J=0.25A.

Output power vs. beam current at U=23.5 kV f=285.2 GHz

0

10

20

30

40

0.15 0.2 0.25 0.3 0.35Current, A

Pow

er,W

0

10

20

30

8.8 9 9.2 9.41-st cavity length, mm

Pow

er, W

94.9

95

95.1

95.2

Freq

uenc

y, G

Hz

95/285 GHzOperating frequency

30WOutput power

Q2

L2

Q1

L1

3400

12 мм,17808 ÷10 мм,

Cavity parameters

TE031(n=3)

ТЕ011(n=1)Operating modes

1.4Pitch factor

0.3ABeam current

25kVVoltage

GYROTRON PARAMETERS:

Free Electron Lasers e

vgrNN

N

S

N

SS

S

SS

N

N

Pout

2/~ γλ d

10120.50.6(200 W av.)

1,700-2,500oscillatorINP&ICKC,Novosibirsk

22-60.50.027120-880oscillatorUCBS, S.Barb.

1.41.430.012100oscillatorTel-Aviv U.

71.760.7, 0.4200, 165oscillatorFOM, Nieuwegein

1002.541095oscillatorNSWC/MRC

0.352.30.190.0015110-150oscillatorENEA Frascati

1500.845150oscillatorColumbia U.

2000.60.51110amplifierILE Osaka

2500613.32000140amplifierLLNL, Livermore

Beam Current, A

Beam Voltage, MV

Efficiency (%)

Output Power, MW

Frequency, GHz

TypeInstitution

Novosibirsk FEL (Budker Institute of Nuclear Physics & Institute of Chemical Physics and Combustion)

Electron bunches: 12 MeV, 10 A, 0.1 ns

Radiation: 120-180 μm 50 ps 5.6 MHz 0.6 MW (peak) 200W (average)

6 working stations

Prospects: 22.5 MHz 40 MeV 5-200 μm

Optoelectronics

Short pulse generation (SPG) –Femtosecond lasers

♦ Fast photoconducting crystals (photoswitches)

♦ Optical rectification (nonlinear crystals)

Quasi-cw narrow-line THz Generation (Difference Frequency Generation – DFG)

♦ Photoswitches

♦ Nonlinear crystals

FemtosecondFemtosecond LasersLasers

λλ ≈≈ 600600--800 800 nmnmττ ≥≥ 50 50 fsfsW W ≤≤ 100 100 nJnJF ~ 100 MHz F ~ 100 MHz <P> ~ 100 <P> ~ 100 mWmWPPpeakpeak ~ 1 MW~ 1 MWFluenceFluence ~ 10 GW/cm~ 10 GW/cm--22

F ~ 100 kHz, W ~ 1μJWith amplyfiers: F ~ 1 kHz, W ~ 1mJ

<P> ~ 500 mW, Ppeak ~ 10 GW GW

Materials used for the Generation of THz Beams:

EO Crystals SC Crystals Organic Crystals

Quartz GaAs DAST LiTaO3 CdTe MOST BaTiO3 GaSb MMONS SrTiO3 GaSe LiNbO3 InSb KNbO3 CdSe LBO ZnSe GaP InP SiC Ge MoS2 Si ZnO KTP ZnTe

Optical crystalsPhotocond.: SOS (silicon-on-sapphire) - τ↑ = 0.1ps, τ↓ = 0.6 ps

InP, GaAs, CdTe, etc.: τ↓ = 50 ÷ 600 ps

Large-aperture photoconductingCerenkov emitter

Zhang e.a., 1990

625 nm, 75 fs, 10 mW; SOS, InP, GaAs, CdTe:spacing 2-5 mm, Vb ~ 100 - 3000 V;high “dark” resistance, strong absorption, fast current rise

Registration: 100 μm SOS-dipolewith lens system

Current Pulse

a) THz pulse at 10 cm distance from the emitter

b) THz pulse at 100 cm distance from the emitter

THz pulse spectrum

Nonlinear generation & detection Nonlinear generation & detection of ultraof ultra--short THz pulsesshort THz pulses

J.A. Valdmanis, et al., APL (1982).M. Bass, et al. Phys. Rew. Lett. (1962).G.A. A’skaryan, Sov. Phys. JETP (1962).

EO crystalInput laser pulseI(t, ω, Δω)

THz pulseETHz(t, Ω)

Dielectric polarization: P(Ω) = χ(2)(Ω, ω+Ω, −ω) E(ω+Ω)E*(ω)

ΔτΔω ≈ 1 t)t(P)t(E 2

2

THz ∂∂

∝P(t)

χ(2)

Δτ

Generation of THz pulse:Optical rectification(second order nonlinear optical effect)

DetectionElectro-optical (EO) sampling(direct E field measurements !!! )

( )τλ

π THzELrnIIII

413

21

21 =+−

EO crystal

THz pulseETHz(t-τ)

Probelaser pulse

I(t)

BSλ/4 WP

I1

I2

Polarization states of the probe pulse

τ - variable time delay

Temporal THz Waveforms1 ps THz pulse

-60

-40

-20

0

20

40

60

EO

Sig

nal (

nA)

1086420Time (ps)

ZnTe sensor

0.1 ps THz pulse

6

4

2

0

-2

-4

EO

Sig

nal (

a.u.

)

20151050Time (ps)

ZnTe sensor

S.Matsura, e.a., 1999

Two-freq. MOPA 850 nm semicond. laser λ1 = 848-853 nm λ2 = 854 nm Power – up to 500 mW

GaAs active area ~ 103 μm2

THz radiation: 0.5 – 2.5 THz up to 0.1 μW (> 10 μW exp.)

DFG - Photoswitches: microstructures low intensities ⇒ low THz output but – cw!

E.Peytavit, e.a., 2002

Ga-AS photodetector

2 cw Ti:Sa lasers (up to 60 mW of total power)

Active area < 102 μm2

THz radiation: max. 0.5 μW at 0.7 THz (essentially less than exp.)

Nonlinear Difference Frequency Generation

● Microstructures are not required ● Potential for high output power

BUT:

● High intensities and strong nonlinearities are necessary

● Quasi-cw regimes instead of cw

Wei Shi e.a., 2002

Pump: Nd:YAG – 10 nsec, 6 mJ, 10 Hz; 6⋅105 W – peak power, 60 mW – av. Tunealbe OPO (pumped by 3-rd harm) – 5 nsec, 3 mJ, 10 Hz

GaSe nonl. crystal: peak int. 17 MW/cm2, (3 mm beam spot, l = 4,7,15 mm

THz radiation: 0.18 – 5.27 Thz, 5 nsec, 10 Hz; peak power 70 W at 1.53 THz

Exotics: Frequency multiplication by multi-element semiconductor structure; THz pulse generation in a laser spark;Quantum cascade lasers.

M.Glyavin, e.a., 2003 (experiment):

About 30x30 DBSs (10x10 mm2)

110GHz/10kW/50 μs GaAs structure 110*3= 330 GHz gyrotron radiation output radiation

30-40 mW

Project: 150-170 GHz gyrotron *(3-5)

- velocity of ionization front0cos/ ϑ=′ cc

THz pulse generation in a laser sparkA.Shalashov, e.a., 2004

сm1≈Lсm10 4−≈a

04

conical lens

0ϑ

THz detect.

c′plasma

E0

( )czt ′− /j

Experiment planned at IAP

receiver

11

Ti:Sa, λ ≈ 800 nm, pulse duration 50-100 fsup to 1 mJ/1 kHz, <P> ~ 0.5 W, Ppeak ~ 10 GW

10pJ/pulse THz, 30 :pressure) (atm. Expected

Quantum cascade lasers of THz frequency range

(Joint 30th Int.Conf. on IR/MM waves & 13th Int.Conf. on THz Electronics, 2005)

1 period – 8 layers, 4.3/14.4/2.4/11.4/3.8/ 24.6/3.0/16.2 nm 230 periods,4.2 К

12 Hz, 1 μs, 2.6 THz

Conclusions

♦ “Terahertz gap” (from the generation point of view) is filled from both sides (vacuum electronics and optoelectronics)

♦ Parameters of needed THz sources are essentially defined by application requirements

♦ Large variety of numerous THz sources are naturally developing not being aimed to some definite application

♦ Practical applications of THz radiation requires development of appropriate detection and registration techniques

♦ A number of impressive examples of THz radiation applications are available based on different radiation sources

♦ The evident prospect for the nearest future: enhancement of main parameters of THz sources and of output radiation; in the application field – the progress from demonstration experiments to a wide use in scientific laboratories